Sound absorbing structure and material



' July 6, 1937. PRUDDEN NG STRUCTURE AND MATERIAL SOUND ABSORBI Filed Vlay 21, 1955 Jay 5..

Patented July 6, 1937 PATENT OFFlCE SOUND ABSORBING STRUCTURE AND MATERIAL Theodore M. Prudden, Hingham Center, Mass.

Application May 21,

6 Claims.

This invention relates to sound absorbing and insulating materials and constructions adapted to be located in the interior of rooms and particularly on a wall thereof, to absorb sound in the room'and also to prevent the transmission of sound through the wall into the room.

The invention is particularly adapted although in no manner restricted to installation in automobile bodies to reduce the noise in theinterior of the body.

The invention is intended as an improvement on the invention of my copending application Serial No.'675,768, filed June 14, 1933, now Patent No. 2,060,241, issued November 10, 1936.

In said copending application I have disclosed a sound absorbing structure comprising a series of closely spaced, sound-vibrating sheets which act iii succession or in stages on the sound each to absorb part of the sound and to transmit the remainder to an adjacent inner sheet.

The present invention is directed to a sound absorbing structure consisting of improved sound absorbing sheets and to a sheet for use in sound absorbing structures.

One of the objects of thepresent invention is the provision of improved means to space the sheets from each other in the structure, the spacing means preferably being a part of the sheet.

sorbing vibratile sheets having means to damp the vibrations thereof, thereby to convert the energy of'vibration into an inaudible form of energy, the damping means preferably being cam- 5 ried by the sheets and arranged in light frictional contact with the face of an adjacent sheet so that the vibrations of both sheets are damped,

the damping means also serving to space the sheets.

Another object of the invention is the provision of a sound-vibrating and absorbing sheet having improved construction wherein it is readily vibrated by the sound impinging upon it and is so arranged as to set up internal friction within it to damp the vibrations and absorb sound.

A yet further object is a sound absorbing sheet composed of loosely felted or interlaced fibres having a smooth stiff face for the above purpose.

Another object is a sound absorbing sheet com.- posed of loosely felted or interlaced fibres havmg a smooth face and loose fibres upstanding from the opposite face, 'and a series of such sheets wherein the smooth face of one sheet is engaged by the upstanding fibres of .the adjacent sheet 55 so that the fibres act as spacing means for the A further object is the provision of sound ab- 1935, Serial No. 22,585

sheets and as means to damp the vibrations of the sheet.

A yet further object of the invention is the provision of a sound absorbing panel comprising a series of relatively freely vibratory sheets enclosed within a cover or facing sheet composed of relatively porous and sound permeable material as felted or interlaced fibres, the facing sheet being so characterized that it is flexible and ca pable of being molded into a suitable shape and of retaining its shape.

A yet further object is generally to improve upon sound absorbing structures and methods.

Fig.1 is a perspective view of a portion of a sound absorbing panel embodying the present invention.

Fig. 2 is a sectional detail 1 2-2 of Fig. 1. v

Fig. 3 is an enlarged sectional view of a single vibratory sheet comprising a part of the invention.

Fig. 4 is a view similar to Fig. 2 but illustrating a somewhat modified construction.

The sound absorbing and sound insulating panel of Figs. 1 and 2 is composed of a series or stack of closely spaced, relatively freely vibratory taken along line sound absorbing sheets Ill, contained between a backing sheet I2, and a facing or cover sheet l4. The sheets are in loose contact, one with another, and all sheets are secured together by lines of stitching it, which lines are relatively widely separated so that the sheetshave broad unattached areas, the stitching being loose so that the sheets are not or but slightly compressed. Lines of adhesive l8 may bind the stitches together and to the backing and facing sheets. A sheet 20 of thin tissue paper, such as stretched crepe paper, preferably is interposed between the facing sheet l4 and the first of the series of sound absorbing sheets I 0. Said paper sheet is secured in the stack by one line of stitching alone, the free end of the sheet being terminated short of the other line of stitching and being unconnected to the other sheets. A paper sheet .22 is interposed between the backing sheet l2 and the last sheet ll! of the series of such sheets and a paper sheet 24 is interposed between the last sheet 10 and the other similar sheets of the stack. The sheet 24 may also be a stretched crepe tissue sheet while the sheet 22 preferably is a glassine paper sheet. The sheets 20 and 24 act mainly as vibratory members to transmit sound. The sheet 22 acts mainly as a sound reflecting sheet which reflectsgoutwardly through the stack the remaining part of the sound that fiects sound that comes from the direction of the backing sheet and prevents the sound from passing into the sound absorbing structure.

The sheet in is composed of cotton wedding or more strictly, cotton lap, which is a thin sheet of loosely felted or arranged cotton fibres. The sheet i0 is provided with a smooth face 26 formed by the application of a sizing material, such as glue size to that face, the sizing bonding the cotton fibres together at the surface and forming, when dry, a smooth somewhat stiff surface. The other face of the sheet has a pile or a fuzzy surface formed by a plurality of looseended fibres 28 which in general upstand more or less from the plane of the sheet and lightly engage the sized smooth face of the adjacent sheet and space the sheets apart. The body of the sheet is formed of more dense but loosely felted or laid fibres lying generally in the plane of the sheet.

The backing sheet i2 is preferably a kraft sheet that is a good sound reflector. The panel is adaptedto be positioned on a wall with the kraft sheet against or facing the wall so that it can reflect back sound that penetrates the wall;

The facing sheet i2 is relatively thick compared with the other sheets and is of a porous sound permeable nature. The sheet consists of loosely arranged fibres, vegetable fibres such as jute preferably being used for economy and also because of the porous or sound permeable nature of the sheet composed of such fibres- The fibres can be felted together or they can be laid,

loosely on an open weave fabric supporting sheet, such as cheese or tobacco cloth, and needled to the fabric.

In the modification shown in Fig. 4 the facing sheet Ila is a so-called roofing felt, that is thinner and denser than the sheet II but sound permeable, nevertheless. In this modification the panel is enclosed within a textile cover 34 of some loosely woven material of the nature of burlap, the ends of the textile material being secured to the backing sheet. The facing sheet Ha can be molded or formed into different shapes to fit various curved surfaces and will retain the shape imparted to it.

The principle embodied in both modifications is the same. The sound absorbing sheets III are relatively free to vibrate under the influence of the sound. The sound waves set the first or paper sheet 20 to vibrating so that it transmits the sound into the interior of the structure, although acting as a check valve to prevent the emission of any unabsorbed sound, due to the difference in phase between the unabsorbed sound and the entering sound and to the mixing in the structure of the various frequency components of the usual sound. The sound transmitted by the entrance sheet 20 sets the first one of the sound absorbing sheets ID to vibrating thereby transmitting further into the structure a part of the sound. This action takes place throughout the structure. The residual sound transmitted by the last sheet ID of the series is reflected by the glassine sheet 22 and the kraft backing sheet l2 and travels forwardly through the sound absorbing sheets which absorb the reflected sound so thatlittle sound remains to beemitted into the room. The reflected and incident sounds are somewhat out of phase and so tend to cancel each other through the vibrating sheets I0.

is passed forwardly therethrough and also re-- Each vibratory sheet l0 absorbs a part of the energy of sound by its vibration and the resistance offered to the vibration by its internal friction-due to the fibrous structure thereof.

Sound energy is further absorbed or converted to an inaudible form of energy by damping the vibration of the sheets by the nappy fibres 28. These fibres are in light contact with the smooth face 26 of the adjacent sheet and so act as friction members to damp the vibration of vboth sheets.

The sound absorbing sheet 10 is to a considerable degree sound impermeable, due primarily to the sized face 26, if the sheet is held against vibration, but when permitted to vibrate is largely sound transmitting and its ability toabsorb sound is due largely to permitting it to vibrate and damping the vibrations. Tests on such a sheet weighing .095 ounce per square foot indicate that the sheet will transmit about 51% of -the total sound impinging thereagainst when the The paper sheets 20 and 24 alone when subjected to sound at 512 cycles per second transmitted roughly 1 5% of the sound when held against vibration and roughly of the sound when free to vibrate.

The weights and elasticities of the resonantly vibratile sheets III, 20 and 22, are selected so as to give the most response tothe sound vibrations, the materials being of different weights and elasticities that are most responsive to the different frequencies that make up a complex sound, if the sound to be absorbed is of a complex nature or has no dominating frequency, so that the panel is high in sound absorption over a broad frequency and also is high in sound absorption in proportion to its thickness.

The panel herein described is composed of materials capable of transmitting approximately of the energy of sound at 512-cycles a second of which transmission at least 25% is by vibration.

I claim:

1. A sound absorbing structure comprising a series of closely spaced relatively freely vibratory sound absorbing sheets stacked loosely one against the other and capable of individual vibration, the sheets characterized by being composed of loosely arranged fibres, some of which upstand from one face thereof and are in light, lolose contact with the face of the next adjacent s eet.

2. A sound absorbing structure comprising a series of closely spaced relatively freely vibratory-sound absorbing sheets stacked loosely one against the other and capable of individual vibration, the sheets being characterized by bein composed of loosely arranged fibres, some of which upstand from one face thereof and are in light loose contact with the face of the next adjacent sheet, the sheets having a relatively smooth surface opposite the fibre surface and upstanding fibres of one sheet being in light engagement with the smooth surface of the next adjacent sheet.

3. A sound absorbing structure comprising a loose stack of sound absorbing sheets capable of individual vibration each having a'nap on one face and an opposite smooth face, the nap of one sheet being in light loose engagement with the smooth face of the next adjacent sheet.

4. A sound absorbing panel comprising a loose stack of sound absorbing sheets of cotton wadding capable of individual vibration each havin a sized smooth face and an opposite fuzzy face, the sheets being arranged with the fuzzy face of one sheet confronting and lightly and loosely 0 contacting with the sized face of the next adjacent sheet. a I

5. A sound absorbing structure comprising a plurality of sound absorbing sheets of cotton wadding capable of individual vibration each 15 having a coating of size on one face thereof and outstanding fibres on the other face thereof,

the outstanding fibres of one sheet being in loose contact with the coating of a confronting sheet, the coated surfaces of all sheets facing'in the same direction.

6. A sound absorbing structure comprising a plurality of relatively freely vibratory sheets, the sheets having free confronting faces and being capable of individual vibration, and means associated with the sheets to damp the vibra tion thereof, said sheets being of a fibrous nature and said damping means comprising fibres of said sheets which are in loose contact with an adjacent sheet, and means connecting said sheets loosely only at wide intervals.

THEODORE M. PRUDDEN. 

